1. Field of the Invention
The present invention relates to an ultrasonic image processing apparatus configured to obtain the motion state of a subject by using an ultrasonic image of the subject acquired with ultrasonic waves, and a method for processing an ultrasonic image.
2. Description of the Related Art
It is extremely important to objectively and quantitatively evaluate the functions of living body tissues such as the myocardium of a heart, for diagnosis of the living tissues. A quantitative evaluation method based on image data of a heart acquired using an ultrasonic imaging apparatus has been proposed.
For example, a method of obtaining motion information such as displacement and strain of a tissue by tracking the speckle pattern of images has been proposed (Japanese Unexamined Patent Application Publication No. 2003-175041). This method, in which pattern matching is performed by using the speckle pattern of images, is referred to as Speckle Tracking (ST). Hereinafter, this method may be referred to as the ST method.
For specific example, in the case of evaluation of the myocardium of a heart, tomographic image data is acquired for each cardiac phase by transmitting ultrasonic waves to the heart. Then, by performing pattern matching by speckle tracking, it is possible to obtain the displacement of the endocardium and the displacement of the epicardium for each cardiac phase.
Then, it is possible to obtain the strain of the myocardium at each time phase based on the displacement of the endocardium and the displacement of the epicardium at each cardiac phase, and furthermore, it is possible to obtain the strain rate indicating the temporal change rate of strain.
This strain rate is a value corresponding to the speed of the heart during expansion. A higher strain rate indicates that a diastolic function is normal. Therefore, the strain rate is used as one index for evaluation of a diastolic function. As described above, evaluation of a heart is performed by obtaining wall-motion information such as displacement, strain and strain rate. Further, it is possible to perform evaluation of a heart by obtaining information such as rotation and twist of the myocardium as wall-motion information.
However, even if the heart is healthy, there may be variations in absolute value of strain depending on cardiac phases. In this case, the variations also affect the strain rate obtained from the strain. Likewise, there may be variations in absolute value of rotation or twist of the myocardium. Therefore, it has been difficult to appropriately evaluate the diastolic function of each part of the myocardium by an evaluation method using the absolute values of wall-motion information such as strain rate.